Switching power sources are widely exploited as the power sources of electric apparatuses used in houses and offices. As methods for the switching power sources, there are known a pulse width modulation method and a method using ΔΣ modulation.
In a switching power source using a conventional pulse width modulation method (PWM), the switching frequency is always constant.
In a switching power source using ΔΣ modulation, the switching frequency changes in accordance with the value of a signal input to a ΔΣ modulator. A conventional synchronous rectification type down converter using a general ΔΣ modulator will be exemplified with reference to a circuit diagram shown in FIG. 15.
In FIG. 15, a signal output from an error amplifier 16 is input to a ΔΣ modulator 1, where the signal is input to an integrator 4 and integrated by it. A signal output from the integrator 4 is input to a quantizer 19, and quantized in accordance with a quantization reference value 6 every cycle (Ts) of a sampling clock output from a sampling clock oscillator 18. The output quantized by the quantizer 19 is negative-fed back to the input of the ΔΣ modulator 1 so as to suppress the quantization error of the signal input to the ΔΣ modulator 1. A 1-bit signal output from the ΔΣ modulator 1 turns on/off a power switch in a voltage converter 9 to smooth an output from the power switch, thereby obtaining a desired output voltage.
The switching power source having such a ΔΣ modulator has the following characteristic. More specifically, when the integrator 4 is of the first order, the count at which a signal output from the ΔΣ modulator 1 changes in a unit time changes linearly with a monotonous increase and decrease having a peak at the center with respect to the output value of the error amplifier 16 serving as a signal input to the ΔΣ modulator 1. This characteristic is disclosed in Japanese Patent Laid-Open No. 2002-300772, and “Characteristics of DC-DC Converter Using ΔΣ Modulation Control”, Yasuhide Imamura, Tetsuro Tanaka, and Hiroshi Yoshida, Technical Report of IEICE, EE2002-78. In the switching power source having the ΔΣ modulator, the cycle of a 1-bit signal output from the ΔΣ modulator 1 serves as a cycle for driving the switch of the switching power source.
From this, the output value of the error amplifier 16 and the switching frequency of the switching power source have a relationship as shown in FIG. 9. In FIG. 9, the maximum value of the switching frequency is ½ the frequency (fs) of a sampling clock output from the sampling clock oscillator 18. The characteristic of a higher-order ΔΣ modulator having a plurality of integrators does not change with a monotonous increase and decrease, as shown in FIG. 9, but tends to similarly increase on average and decrease on average.
In the switching power source having the ΔΣ modulator, the switching frequency is lower than the maximum switching frequency (=½·fs) determined by the sampling frequency (fs) of the ΔΣ modulator in the range of a voltage input to the ΔΣ modulator. For this reason, the switching loss can be reduced. By using this feature, the switching frequency at the highest speed can be set higher than that in PWM control. This is advantageous since the control frequency can be set high.
The operation of the power source will be described more specifically. In a steady state in which no output state of the switching power source changes, a switching frequency (fsw) of the switching power source decreases, thus reducing the switching loss. In a transient state in which an output from the power source changes, the switching frequency (fsw) of the switching power source increases, thereby enabling a quick response to an abrupt change in load or output voltage. Of switching power sources having ΔΣ modulators, a diode rectification type switching power source as shown in FIG. 10 can reduce the switching frequency at a light load. This switching power source can greatly increase the efficiency at a light load.
However, the switching power source having the ΔΣ modulator suffers the following problems.
As shown in FIG. 9, in response to an output from the error amplifier 16, the switching frequency of the switching power source becomes zero at the upper and lower limits of an input which can be modulated. The switching frequency has a triangular-shaped output characteristic with which the switching frequency reaches its peak at the median of an output from the error amplifier 16. In other words, the switching frequency increases from the lower limit value of an output from the error amplifier 16 to the median, and decrease from the median to the upper limit value.
In the conventional synchronous rectification type switching power source shown in FIG. 15, when its output state does not change, the output voltage of the error amplifier 16 is determined by the ratio of a voltage (Vin) at an input voltage terminal 11 and a voltage (Vout) at an output voltage terminal 12. If the output value of the error amplifier 16 always keeps a value around the center of the triangular shape shown in FIG. 9 in accordance with the relationship between the input voltage (Vin) and output voltage (Vout) of the switching power source, the switching frequency is always high. Hence, no advantage of the switching power source using the ΔΣ modulator can be obtained.
Even in the diode rectification type switching power source shown in FIG. 10, as the load current increases, the output value of the error amplifier 16 may vary around the median of the output range of the error amplifier 16 that can be modulated by the ΔΣ modulator 1 (see FIG. 9). Thus, the switching count may increase at a light load, thus decreasing the power conversion efficiency due to an increase in the switching loss, (see Japanese Patent Laid-Open No. 2002-300772).
In order to overcome these drawbacks, Japanese Patent Laid-Open No. 2002-300772 discloses a switching power source using a ΔΣ modulator in which a frequency control circuit is arranged in the ΔΣ modulator to control the frequency (fs) of a sampling signal. This configuration adjusts the switching frequency.
FIG. 11 shows a change in switching frequency upon a change in sampling frequency (fs).
Japanese Patent Publication Laid-Open No. 2002-64383 discloses a function of inhibiting re-inversion of a signal output from a ΔΣ modulator when the number of clocks output upon inversion of the output signal is equal to or smaller than a preset value N (N≧2). By using this function, the power conversion efficiency can be increased by preventing an excessive increase in switching frequency, and the above-described drawbacks can be overcome.
However, the method disclosed in Japanese Patent Laid-Open No. 2002-300772 poses the following problems. More specifically, when the sampling frequency (fs) is changed at a given rate, as shown in FIG. 11, the switching frequency over the entire voltage range of an output from the error amplifier which serves as a signal input to the ΔΣ modulator changes at the same rate. Thus, if a power source output frequently changes, the sampling frequency (fs) must always be controlled in accordance with the power source output. As a result, it becomes complicated and difficult to control the sampling frequency (fs) in accordance with various situations, such as a case where the power source output abruptly changes at high speed.
If the switching frequency is decreased by decreasing the sampling frequency (fs), the control frequency also decreases as a whole, quantization noise becomes large, and the control accuracy decreases.
According to the method disclosed in Japanese Patent Publication Laid-Open No. 2002-64383, the switching frequency is defined by the clock count (N). For this reason, in order to finely adjust the switching frequency, the clock frequency, which determines the inversion period of a signal output from the ΔΣ modulator, must be set much higher than the switching frequency.
As described above, the ΔΣ modulation type switching power source needs to quickly respond to a change in power source output at high efficiency over a wide output voltage range without changing the sampling frequency (fs) or complicating the configuration or control.